This invention generally relates to a key telephone system, more particularly a key telephone system having a plurality of data terminal equipments, such as personal computers, as extensions.
In a prior art system of this type, a data terminal used to answer to an incoming call from an office line has been predetermined. When an incoming call is received from an office line a call signal is sent to the predetermined data terminal, and due to the answer of the data terminal a connection is established in the system via a modulator-demodulator (MODEM) between the data terminal and the office line for starting a speech communication.
Such prior art system has a problem that a calling party can not arbitrary select a data terminal the calling party desires to be called.
As an approach for solving this problem, a service called a PBX dial-in can be used. According to this PBX dial-in service, since after receiving an incoming call, an extension selection signal is sent from a telephone office to the system, the system can select a data terminal to be called. However, all systems can not always enjoy the PBX dial-in service- More particularly, since an extension selection signal of the PBX dial-in system is a multi-frequency (MF) signal, only a system constructed according to a MF signal specification can enjoy the service so that many systems constructed according to the dial pulse specification can not enjoy the PBX dial-in service.